This invention relates in general to cryopumps and in particular to a new and useful method and apparatus for rapidly regenerating a self-contained cryopump.
The invention relates to cryopumps serving the purpose of producing medium and high vacuum in apparatus used for vacuum processes on an industrial scale. Recently, for some years, cryopumps are employed for this purpose at a growing rate, since they not only have a very high specific suction capacity, but also are capable of producing a "clean" vacuum, namely free from hydrocarbons and having low final pressures. Since, in contradistinction to delivery-type vacuum pumps, the evacuated gases are stored in the cryopump, a regeneration is needed from time to time. The invention deals specifically with this problem.
Cryopumps are employed, for example, in apparatus for producing thin layers in a cathode sputtering process, operating with a relatively large argon thruput and within the region of 10.sup.-3 to 10.sup.-2 millibar. To obtain in program controlled units reproducible results and layers of good quality, the partial pressure of the other residual gases, particularly of hydrogen, must be kept as low as possible in the coating chamber.
A satisfactory capacity for exhausting hydrogen, however, is important also in other vacuum processes, and in the vapor deposition under high vacuum, for example where metals are evaporated from crucibles containing evaporative substances or through wall reactions in the evaporation chamber, considerable hydrogen amounts may be set free. But, just for hydrogen, and also for helium, the storage capacity of conventional cryopumps is low.
To operate so called self-contained cryopumps, i.e. such working without a supply of a coolant from the outside, mostly cryogenerators are now used. The operation of which is based on either the Stirling cycle or the Gifford-McMahon cycle. To produce the low temperatures needed for condensing the permanent gases, frequently two series-connected cryogenerator stages are provided. On the cryocondensation surfaces connected to the first stage, which will be termed high-temperature stage (HT stage) in the following, the gases easier to condense, such as water vapor, CO.sub.2, and higher hydrocarbons, undergo condensation. In the HT stage, the temperature mostly ranges from 70 to 120 K. This stage cools at the same time the radiation screen for the second stage which will be termed low-temperature stage (LT stage) in the following. On the condensation surfaces which are connected to this LT stage, gases, such as Ar, O.sub.2, and N.sub.2 are either frozen out, or, such as H.sub.2, He and Ne are fixed by cryosorption to a sorbent, for example activated carbon, In the LT stage, the temperature mostly ranges between 15 and 20 K.
The temperature, establishing on the cryosurfaces of the two stages is determined by the refrigerating capacity available at the respective stage, and by the enthalpy of the exhausted gases and the thermal flux through radiation and heat conduction from the ambience.
The equilibrium pressure of a condensed and sorbed gases, such as hydrogen, is a function of the temperature establishing in the LT stage. Even though at a temperature of 20 K., H.sub.2 has an equilibrium pressure of approximately 1 bar, the partial pressure of hydrogen can be lowered below 10.sup.-6 millibar by cryosorption in activated carbon which is glued on the LT cryosurface. However, the amount of hydrogen removable by pumping is limited. It depends on the amount and temperature of the sorbant, and on the amount of gases which are sorbed simultaneously or have been sorbed earlier. After a certain time, the sorbant becomes saturated and the equilibrium pressure of the hydrogen starts rising. Then, it is necessary to regenerate the sorbant by baking it out. Up to the present time, this was possible only upon stopping the operation of the cryogenerator.
To avoid a premature exhaustion of the sorbing capacity and thus heep the surface temperature of the sorbant, which is codeterminative for the equilibrium pressure in dynamic pumping processes, as low as possible, the LT cryosurface areas which are covered by the sorbant must be so disposed that they are protected against irradiation from surfaces having a higher temperature, and that all the gases, except He and H.sub.2 are condensed with a high probability prior to arriving at the sorbant.
Even while complying with this precondition, experience has shown, at least in instances where, unlike in cathode sputtering for example, not too large amounts of gases are exhausted, that as a rule, the hydrogen must be removed already at an instant at which the equilibrium pressure of the other gases on the LT cryosurface has not yet exceeded permissible values.
As mentioned, an exception is the use of a cryopump in a cathode sputtering process. There, mostly so large gas amounts must be condensed that finally a regeneration becomes a necessity, because of the temperature gradient building up on the condensate layer, or of the clogging of the intermediate spaces between the condensation surfaces. In both instances, up to the present, the operation of the vacuum apparatus must be stopped for regeneration, which of course enters into operating costs and is to be minimized.
According to prior art methods, to effect a regeneration, the cryopump is separated from the vacuum apparatus by means of a high-vacuum valve, and then stopped. The result is that the cryosurfaces heat up, initially slowly due to thermal irradiation from the ambience, and then faster due to the heat conduction of the gas again evaporating from the condensation surfaces, up to the room temperature. The gases set free are evacuated by the fore-vacuum pump which, besides, is needed for initially evacuating the vacuum apparatus. Condensed water also re-evaporates, yet becomes partly absorbed on the inside surfaces of the cryopump.
The cryogenerator can then be cooled down again by restarting its operation, as soon as a pressure of about 0.1 millibar is reached again in the cryopump. This lowers the partial pressure of water vapor very rapidly to values below 10.sup.-3 millibar. Since the residual gas is composed substantially of water vapor, the thermal conductivity is then small relative to the thermal irradiation, so that the greatest part of the refrigeration capacity is available for cooling down the cryogenerator and the cryosurface.
The time needed for regeneration includes the heating period and the cooling period. The heating period is determined by the enthalpy of the condensed gas amount, and by the mass of the HT and LT stages and the respective cryosurfaces. In processes involving a high thruput of gases, such as in cathode sputtering units, the first named factor may be determining, while the other factors are predominant mostly in apparatus having a small thruput of gases.
The cooling time depends substantially on the cryopump masses to be cooled and on the refrigerating capacity of the two stages in the respective temperature ranges. As a rule, a regenerative cycle of a self-contained cryopump takes several hours.